Thermal batteries are the reserve power “of choice” on board many weapon and defense systems, due to their very long shelf life (about 25 years). The thermal batteries are kept in an essentially frozen state, until activated by heating. Within milliseconds of being heated to operating temperature, thermal batteries can produce very high pulse power outputs. Power generated by such batteries is utilized for guidance, communication, and arming of weapon and defense systems. Accordingly, thermal batteries play a critical role in our national defense.
Fiber mats of boron nitride (BN) suitable for use as separators in lithium-sulfide thermal batteries have been described by Hamilton, U.S. Pat. No. 4,284,610 and Maczuga, U.S. Pat. No. 4,354,986. The production of BN fibers is described by Economy, U.S. Pat. No. 3,668,059. BN separators exhibit structural stability (compressive strength), small interstices (useful as a particle barrier), and can hold a large volume fraction (65-85%) of a molten halide electrolyte. Because of its unacceptable high production cost (e.g., due to difficult, high temperature fabrication), and difficulty in initiating wetting with molten halide, the use of BN as a separator has been abandoned in favor of high-surface area MgO powder-based separators. Pressed-powder MgO separators are relatively inexpensive, have chemical stability, and can immobilize 65-85 volume % of electrolyte within the interstices of the pressed powder. A significant drawback of MgO separators is the limited structural stability of the material, which results in undesirable limits on thinness of the separator that can be obtained with this material. A MgO powder separator when combined with molten electrolyte, is a paste at thermal cell operating temperatures.
Attempts to prepare MgO fibers have resulted in highly frangible products, which are reduced to particles under compressive loads. U.S. Pat. No. 4,104,395 to Frankle teaches that impregnation of organic fibers with precursor materials can form mineral fibers after high temperature processing (e.g., 1400° C.). Smith et al. (U.S. Pat. No. 4,992,341) teach production of fiber-like sheets of MgO, by layering a sheet of MgO powder in threads in a combustible binder, and then sintering the layered material to decompose the binder and form a fiber-like MgO sheet structure.
Due its frangible nature, formation of a porous MgO structure with sufficient compressive strength for use in thermal batteries is limited to materials having only about a 50% open volume fraction available for receiving a working fluid, such as a molten halide electrolyte. For example, Briscoe et al. U.S. Pat. No. 5,714,283 describe formation of MgO structure by sintering a MgO precursor (e.g., a soluble magnesium salt of an organic acid) on a microporous sintered metal screen support. The resulting sintered MgO films, having a thickness of about 3-25 mils, but have only about 20-50% open volume for incorporation of an electrolyte. The low open volume of such materials imposes performance limitations in thermal battery applications, especially if there is structural disintegration (e.g., due to mechanical stressed during manufacture, etc.).
In thermal battery technology, the trend has been toward development of higher power density. The design approach for this has typically involved producing thinner cells. Thermal batteries are produced from stacked cells consisting of pressed powder wafers in the following repeating order: (a) a heat pellet, (b) a Li-alloy negative electrode (anode), (c) a porous separator (e.g., MgO) containing a meltable electrolyte salt, and (d) a FeS2 positive electrode (cathode). Each wafer typically is about 1 mm (about 39 mils) thick. Battery performance could be improved by using a thinner separator, if suitable materials were available. Thinner MgO separators are impractical due to the physical strength limitations inherent in the MgO pressed-powder materials.
The separator component of a thermal battery physically separates and ionically couples the anode and the cathode in each cell of the battery. Ideally, a separator should have a relatively high capacity for an electrolyte and have connected porosity for high performance. Added characteristics of importance include dimensional stability and flexibility. In one application, wafer thin separator components limit battery pulse power to about 5.5 kW. A thinner separator would boost the proportion of active materials (electrolyte) in the battery, and thus boost power output. Unfortunately, MgO powder wafers have limited handling strength, and generally must be at least about 1 mm in thickness for practical use in thermal batteries. Thinner MgO tends to crack or break, thus compromising the integrity of the entire battery. Larger diameter wafers exacerbate the handling problems. Because of this, MgO powder wafers must have a substantial thickness to be of practical use. In addition, volumetric changes of the active electrolyte material tends to distort the electrolyte/separator interface, which leads to cell shorting.
The prevailing construction and chemistry of a state-of-the-art thermal battery has been around for about 25 years. It uses Li-alloy and metal sulfide electrodes, with a lithium halide salt as the electrolyte. The salt becomes molten upon heating. As noted above, the battery is composed of a stack of wafers of pelletized powders. Wafer fabrication and battery assembly involve substantial hand labor, partly due to the frangible nature of MgO separators. The wafer pressing operation has received some automation, but battery assembly relies on hand stacking of components.
Current thermal battery manufacturing employs uniaxial powder pressing technology to form active cell components. The thickness, diameter, and overall geometry (parts are typically cylindrical) of the wafers are limited by the uniaxial powder pressing process. The thickness obtainable for uniaxially pressed wafers for thermal batteries ranges from approximately 1 mm to about 10 mm. Production of thinner or thicker parts is notably more difficult, and commonly results in low yields, and therefore, higher costs. Thinner wafers require precise, even die loading, while thicker wafers require the use of organic binders to distribute the applied pressure evenly. Similarly, large diameter wafers are difficult to uniaxially press due to increasingly larger processing equipment required to provide the necessary mechanical loads to form the wafers—typically greater than about 10,000 pounds-per-square inch (psi). These limitations preclude many advanced battery designs.
For electrode pellet manufacture, a high tonnage press typically is required to achieve 50 volume % active electrode material loading. A portion of the electrolyte salt generally is combined with the electrode material to aid in the formation of suitable cold-pressed pellets. The metal sulfide electrode material, FeS2, is a very hard material and does not compact well on its own. Typically, the pressed electrode uses FeS2 coated with electrolyte salt to facilitate the powder compaction. The resulting cold-pressed pellet generally comprises about 50 volume % FeS2, about 30 volume % electrolyte salt, and a void volume of about 20 volume %. An unpressed powder layer would typically have a void volume of about 50 volume %. To achieve the desired 50 volume % active material loading, the high tonnage press must displace about 30% of the void volume that is needed for the electrolyte salt. This is crucial, in that unpressed electrodes with a 20-30 volume % loading of electrolyte exhibit poor performance (e.g., low energy density and low power output).
The separator material used in previous molten salt thermal batteries is pressed from a high-surface area MgO powder. MAGLITE® S or MAGLITE® D (Calgon), and more recently MARINCO® OL (Marine Magnesium Company) magnesium oxide mixed with electrolyte salt, have been the materials of choice for pressed powder separators. Alternative materials have been investigated, but only the pressed-powder MgO/salt separator has found commercial application.